The invention relates to lasers, in particular, a laser source able to emit several different wavelengths in the infrared. Laser sources that are able to emit several wavelengths in the various atmospheric transmission bands are needed for numerous applications, such as LIDARs, detecting atmospheric pollutants, or optronic countermeasures.
The common element for this type of laser developed today is a solid fixed or tunable wavelength source associated with nonlinear optical components, such as an optical parametric oscillator (OPO) or Raman converter.
The problems encountered in developing these lasers relate to the spatial quality of the beams obtained, the average pulsed energy or power performances, and the total efficiency expressed as usable laser power generated in the spectral bands to be covered relative to the electric power injected into the pumping diodes. Very often, at the output of an OPO converter, one of the two wavelengths generated (signal or idler) is outside the spectral range investigated. Thus, when the wavelength does not exceed 2 μm, an OPO designed to cover the II band, namely 3 to 5 μm, cannot emit in the upper part at about 5 μm and in the bottom part at the same time. When, for optronic countermeasure applications, the goal is to cover the I band (about 2.1-2.2 μm) and the II band (about 4.1/4.2 and 4.6/4.7 μm) with a pumping laser followed by an OPO, the pumping wavelength must be above 2 μm. Today, for these applications, two diode-pumped solid source designs are preferred.
A source based on a neodymium laser emitting at about 1 μm, and associated with 2 OPOs in a cascade, is needed to reach the II band. For example, Nd:YVO4 emitting at 1.06 μm at a repetition rate of 5 kHz followed by a first OPO (PPNL, PPKTP, KTP, KTA, etc.) that supplies two waves at 2.18 μm and 2.06 is needed. λ1=2.06 μm can be considered as being in the I band. The wave at 2.18 μm pumps a second OPO (ZGP for example) so that two wavelengths can be obtained, namely 4.1 and 4.6 μm in the II band. The theoretical efficiency is 18% at the output of the first OPO for λ=2.18 μm assuming that the efficiencies are near-identical for the signal and the idler. By comparison to the pump beam at 1.06 μm, the beam at 2.18 μm has a spatial profile of distinctly inferior quality. At the output from the second OPO for the two II band wavelengths, λ2 at 4.1/4.2 μm and λ3 at 4.6/4.7 μm, the total efficiency is less than 9% and the profiles of the emitted beams have deteriorated still further.
Another at least equally advantageous solution is based on a Tm—Ho source emitting at 2.09 μm associated with a single OPO to emit in the II band. At 2.09 μm, the beam quality is excellent (M2<1.2) and the efficiency is over 20%. However, in a ZGP OPO emitting at 3.83 and 4.6 μm, one of the two wavelengths, λ2=3.83 μm, is not ideally placed for optronic countermeasure applications. Moreover, the ZGP crystal has, at 2.09 μm, depending on the quality, an absorption coefficient of between 0.03 and 0.1/cm. This Tm:YLF→Ho:YAG source design as the pumping source for an OPO has two other drawbacks.
The Ho:YAG pumping wave supplied by Tm:YLF at 1.91 μm is close to a water vapor absorption line, which leads to intensity fluctuations. For a military application, the Tm:YLF source must be placed in a dry-air enclosure. Replacement of Tm:YLF by a thulium doped silica fiber laser makes the assembly more stable, but has a lower efficiency because the pumping efficiency of thulium at λ=0.793 μm is not the same in silica as in a YLF crystal. Moreover, the length of the Ho:YAG crystal laser pulses in the Q-switched mode varies considerably with the repetition rate. The length increases from 30 ns to 120 ns for a rate increasing from 10 kHz to 50 kHz. Thus, the OPO, placed behind the Ho:YAG pulsed source, has a behavior that varies considerably with the repetition rate. In the Tm:YLF→Ho:YAG source, the Tm:YLF crystal remains fairly fragile despite the use of composite crystals that allow the fracture limit to be pushed back to 15 kW/cm2.